This invention relates generally to printing devices. More specifically, the present invention relates to the detection of invisible ink applied on a print medium.
Printing mechanisms, e.g., printers, plotters, photocopiers, facsimile machines, etc., are often implemented to record information, e.g., text or graphics, on recording media such as paper, fabric, textile, and the like. In performing recording operations, marking implements, e.g., printheads, are often used to apply an ink onto the recording media.
The positional accuracy of the marking implements as well as the nominal inking density and/or flow volume, are significant factors in assuring that the ink is applied onto the recording media in a desired manner. In an effort to maintain these factors within predetermined operating conditions, conventional printing mechanisms may perform calibration operations. These calibration operations typically entail the printing of a test pattern by the marking implements along with a scanning operation of the test pattern.
The scanning operation is typically performed with optical sensors having a light emitting diode (LED) that emits a light over the test pattern and a sensor that detects the light reflected from the test pattern. Based upon the reflected light patterns, characteristics of the applied ink such as placement and drop volume, may be determined. Once these characteristics are determined, the marking implements may be calibrated, e.g., adjusting the firing time of ink from the marking implements during printing passes to more accurately position the ink on the recording medium, varying the amount of ink fired from the marking implements, etc.
Conventional optical sensors may be unable to detect colors that are similar to the recording medium, e.g., yellow on white paper, etc. Moreover, conventional optical sensors are typically unable to detect inks having no color, i.e., xe2x80x9cinvisible inkxe2x80x9d. Throughout the present disclosure, xe2x80x9cinvisible inkxe2x80x9d generally refers to observations of ink coated onto some particular printing medium under some particular illumination. In addition, xe2x80x9cinvisible inksxe2x80x9d are inks having colors that do not provide adequate contrast, relative to the recording medium background without the color, for adequately reliable detection by the sensor. It should be understood that ordinarily, invisible ink may be visible to the normal human eye, even though the sensing system may be unable to distinguish it well from the print medium background. In addition, some applications may make use of ink that is invisible to the human eye as well.
In one respect, conventional optical sensors may be unable to distinguish between these invisible inks and the recording medium color, which may be white or some other light color. In these situations, it is generally known to print a relatively dark colored ink, e.g., magenta ink, in an area fill. The area fill containing the colored ink is printed upon with the invisible ink, which mixes with the colored ink. The resultant mixture causes the colored ink to change its color, e.g., renders the colored ink to become brighter. A standard optical sensor may then be implemented to detect the differences in the color of the colored ink in the fill area to determine where the invisible ink has been applied.
One problem associated with the above-described technique is that it requires the use of the dark colored ink to determine the locations of the invisible ink applications. This generally results in a waste of the dark colored ink as well as the certain portions of the print medium where the dark colored ink is applied.
Alternatively, it is generally known to add a so-called xe2x80x9cmarkerxe2x80x9d element to the invisible ink. The marker typically consists of an ink that may comprise a color that is invisible to the human eye. However, systems employing this type of marker element are relatively complex and are thus associated with higher operating costs. In addition, they require the use of additional hardware, e.g., additional optical sensors that are capable of detecting this type of marker.
According to an aspect, the present invention pertains to a method for detecting invisible ink drops on a print medium. In the method, one or more drops of the invisible ink is fired from at least one nozzle of an ink ejection element onto a first intended location on the print medium. An area encompassing the first intended location is illuminated and changes in light reflectance are detected around the area. In addition, an actual location of the one or more invisible ink drops is determined in response to detected changes in the light reflectance.
According to another aspect, the present invention relates to a system for determining presence of invisible ink drops on a print medium. The system includes an optical scanner operable to detect specular reflections from the print medium and locations on the print medium containing the invisible ink drops. The system also includes a controller operable to detect changes in the specular reflections to determine the locations of the invisible ink drops.
According to a further aspect, the present invention relates to a method for detecting invisible ink drops on a print medium. In the method, an optical detector is scanned over the print medium in an area believed to contain the invisible ink drops. In addition, changes in reflectance of the print medium are detected in the area.
In comparison to known printing mechanisms and techniques, certain embodiments of the invention are capable of achieving certain aspects, including, relatively inexpensive and simple manner of detecting the presence of invisible ink drops applied on a print medium, a system and method for detecting invisible ink drops without requiring the use of a marker, and the ability to print invisible ink drop test patterns on various sections of the print media other than waste areas. Those skilled in the art will appreciate these and other advantages and benefits of various embodiments of the invention upon reading the following detailed description of a preferred embodiment with reference to the below-listed drawings.